In recent years, in the field of audio-visual apparatuses typified by notebook computers, small lithium-ion batteries with high output power have been used. In order to obtain a predetermined output and a predetermined capacity for a notebook computer, a battery module is used that contains series-connected parallel blocks, each containing parallel-connected cells.
The parallel blocks are connected in series via lead plates and the lead plates are connected to a protection circuit. The parallel blocks are hard to connect to both sides of the lead plate in view of welding. Thus in the related art, parallel blocks to be connected in series are welded to both ends of a surface of a lead plate and then the lead plate is bent, so that a space-saving battery module is formed.
FIG. 10 is a structural diagram showing an example of a battery pack containing cells connected in two rows and three columns according to the related art. Three parallel blocks 9 are connected in series via lead plates 2a and 2b to form a battery module 8. Each of the parallel blocks 9 contains two cells 1 connected in parallel. The battery module 8 is connected to a protection circuit board 3 via the lead plates 2a and 2b. 
In a lithium-ion battery, in order to secure safety in any wrong use of the battery module 8 containing the connected cells 1, the protection circuit board 3 is provided in a battery pack 10 and prevents overcharge, overdischarge, and overcurrent. Particularly, in order to prevent overcharge having the highest degree of danger, the parallel blocks 9 are connected in series, each containing the parallel-connected cells 1.
The parallel blocks 9 are connected to the protection circuit board 3 via the lead plates 2a and 2b. The voltages of the parallel blocks 9 are monitored by the protection circuit board 3 to prevent overcharge.
FIG. 11 is a development showing a connection state of the parallel blocks according to the related art. As shown in FIG. 11, first in the battery module 8, the electrodes of metal jacket bottoms 1b are connected to the lead plate 2a, the metal jacket bottoms 1b serving as the negative electrodes of the two cells 1 constituting a parallel block 9a. Further, the electrodes of caps 1a are connected to one end of the lead plate 2b, the caps 1a serving as the positive electrodes of the two cells 1 constituting the parallel block 9a. Moreover, the electrodes of metal jacket bottoms 1b are connected to the other end of the lead plate 2b, the metal jacket bottoms 1b serving as the negative electrodes of the two cells 1 constituting an adjacent parallel block 9b. 
Next, the electrodes of caps 1a are connected to one end of a lead plate 2b′, the caps 1a serving as the positive electrodes of the two cells 1 constituting the parallel block 9b. Further, the other end of the lead plate 2b′ is connected to the electrodes of metal jacket bottoms 1b serving as the negative electrodes of the two cells 1 constituting an adjacent parallel block 9c. Moreover, a lead plate 2a′ is connected to caps 1a serving as the positive electrodes of the two cells 1 constituting the parallel block 9c. 
The lead plates 2a, 2b, 2a′, and 2b′ are connected to the cells 1 by spot welding. A parallel-gap welding rod 12 is pressed onto the lead plates 2a, 2b, 2a′, and 2b′ at the long and short dashed lines of the cells 1 while applying a large current to the lead plates, so that the temperature of the metal is increased to the melting point of the metal and welding is performed by using heat generated by the contact resistance and the specific resistance of the metal.
The cells 1 and lead plates 2a, 2b, 2a′, and 2b′ connected thus constitute the battery module 8 in which the lead plates 2b and 2b′ are bent into U shapes and the cells 1 are formed in two rows and three columns. The fabricated battery module 8 is connected to the protection circuit board 3 by soldering one ends of the lead plates 2a, 2b, 2a′, and 2b′, and then the battery module 8 is accommodated in a case.